Signal transmission



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Filed May 14, 1957 ll bio Xghk ww# March 15, 1960 F. vlLBlG SIGNALTRANSMISSION 4 Sheets-Sheet 4 Filed May 14, 1957 United States Patent aSIGNAL TRANSMISSION Friedrich Vilbig, Cambridge, Mass., assignor to theUnited States of America as represented by the Secretary of the AirForce Application May 14, 1957, Serial No. 659,183

8 Claims. (Cl. 179-1555) (Granted under Title 35, U.S. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the United States Government for governmental purposes withoutpayment to me of any royalty thereon.

l This invention relates to methods and systems for communication,particularly methods and systems for analyzing speech by converting itinto electrical signals and then causing the signals to function in suchmanner as to reproduce the speech. As used herein, the term speechembraces all vocal utterances.

Speech-band compression aims to miniaturize the frequency-band withoutimpairing the desired informational content. Spectroscopie investigationof speech has shown a pitch-harmonic line-spectrum for vowels and anoiselike spectrum for turbulent consonants (excitation function). Theenvelope of both types of spectra represents the system-function.

Voice coding systems may achieve a compression factor of 10 to 20. By aspectrum analysis on the transmitter side, a pitch frequency isextracted and a system-function is derived. Both are transmitted to areceiver where speech is synthesized.

Speech contains audible frequencies up to about 16 kc. Speechtransmission will be nearly perfect in respect to fidelity andintelligibility if a 16 kc. wide band is used for transmission, and ifthere are no distortions, either linear or non-linear. Conversation willsound as though the persons are facing each other. Due to economical ortechnical consideration, the transmission band is reduced to about 3 kc.by low-pass filtering. By this limitation, the intelligibility, is notinuenced very much, but it would be decreased considerably upon furtherlimitation to lower frequencies.

To arrive at a better utilization of the transmission lines thespeech-band is compressed without losing the necessary informationcontent. Ideally performed, the process should not be detectable by thelistener. Such a compression is obtainable by a scan coding process atthe transmitter, and a scanV decoding at the receiver side. This processrequires at the transmitter side an analyzer and on the receiver side asynthesizer for speech.

There are other advantages connected with speechband compression such asa reduction of noise and a certain security against undesired listening.

In order to achieve speech-band compression certain characteirstics ofspeech vibrations may be utilized. An analysis of speech vibration inthe case of a vowel will show a typical repetition of certainconfigurations of vibration where the repetition frequency is calledpitch frequency. lf unvoiced sounds are considered, a more or lessirregular vibration is indicated.

In an inspection of the frequency-spectrum distribution of speech, inthe case of vowels, a large number of harmonics of the pitch frequency,the fundamental frequency excited by the larynx, is indicated. Thismeans we have a line spectrum, while non-vowel sounds cause a noisespectrum. The amplitudes are represented by difvowel case is energyconcentration in certain frequency areas which are called the formantareas.

Generally, a vowel has three significant formants and the position andamplitude of the formants are characteristic for a particular vowel.

The analysis of speech vibration results, as already mentioned, in somephysical characteristics; in the case of vowels, there is a harmonicline-spectrum, where the lines have the distance of pitch frequency. Inthe case of nonvowel sounds, there is noise spectrum. Last, there arethe energy distribution and the formants indicated by the envelope.Furthermore, an analysis produces indications` about the electricresponse of the time functions of the sounds which can be used. Forexample, discrimination may be achieved between continuous consonants ass and f which can be produced over a longer time interval compared withsuch stop sounds as p, t, and k which have pulse-like time functions.

It is possible to describe the speech vibrations by merely physicalvalues, such as frequency, amplitude, energy content, time functions andso on. Methods using exclusively these physically measurable values forspeech compression may, therefore, be called physical methods.

A consideration of instantaneous pictures of a speech spectrum displaystwo typical extremes; a harmonic linespectrum and a noise-spectrum. Theone which is produced depends on the excitation of the speech sounds.Beyond this, the envelope of the spectrum is of importance, due to theresonant structure of the sound source system. The spectrum curvecontains the characteristic of the excitation function and of thesystemfunction. In sound-excitation there is also a distinction betweenvoiced sounds or vowels and unvoiced sounds or non-vowels. If a vowel isproduced, the air current coming out of the lungs will be rhythmicallymodulated by the vibration of the larynx. The fundamental frequency ofthe so excited air pulses has already been defined as pitch frequency.It varies for different persons, differing in its frequency andinflection. In the spectrum of vowels we find a number of frequencylines which are harmonics of the pitch frequency. This is the picture ofa harmonic line spectrum. Also, we may say the appearance of a harmonicline spectrum is characteristic for a vowel excitation.

Uttering a non-vowel sound, e.g., a hissing sound, will not excite thelarynx, as may be perceived. A hiss will be produced by the air currentwhich is represented by a noise spectrum. In general, we may say theappearance of a noise spectrum is characteristic for a nonvowelexcitation.

The transmission of a so-called pitch-hiss-signal indicates either thepresence of a voiced sound or vowel with the pitch frequency asfundamental in the line spectrum or of an unvoiced sound with a noisespectrum. Beyond the pitch-hiss discrimination, the pitch-frequencysignal is another important excitation characteristic. It indicates thefrequency and the alteration in pitch frequency. But it is notabsolutely necessary to transmit the pitch frequency too, since it ispossible to add an artificial pitch Y frequency which is somewhatdifferent from the original pitch on the receiver side. This would avoidusing a frequency-band for transmission of the pitch signal, without anyimportant loss of intelligibility. Also, it is possible to combine thepitch-frequencysignal with theA pitch-hiss-signal, for instance, by theassumption thatk sented by the spectrum envelope. The so-called formant;

Patented Mar. 15, 1960 ranges are resonant areas which result from theresonant positions of the different cavities in the vocal tract. Inspeaking vowels, the positionof the tongue, the lips, and so forth, willchange the cavity resonances. If these cavities come to vibration byexcitation of the pitch modulated air pulses, direct and coupledresonance frequencies will be excited. These build up the alreadymentioned formant frequencies. Each formant frequency will appear as adamped oscillation periodically excited by the pitch-period. Thisprocedure can also be seen Linder the aspect of formant frequenciesmodulated by the pitch frequency and its harmonics, which result in aline spectrum.

In non-vowel sounds the excitation of the different cavities of thevocal tract is accomplished by a continuous air current modulated bynoise. This noise modulation results from friction and turbulentexcitation in the mouth, especially at the teeth. Very high resonanceswill appear in speaking s and f sounds, for instance, since .the smallslot between teeth and lips is an acoustical resonator of highfrequency. The resuit is that also in nonvowel sounds, the envelope is apicture of the systemfunction.

It has been seen that speech contains a pitch-harmonic line-spectrum forvowels and a noise-like spectrum for turbulent consonants (excitationfunction). The envei lopes of both types of spectra represents thesystem-function. Also it has been shown that the transmission of aso-called pitch-hiss-signal indicates either the presence of a voicedsound or vowel with the pitch frequency as fundamental in theline-spectrum or of an unvoiced sound with a noise spectrum. Beyond thepitch-hiss discrimination, the pitch-frequency signal is anotherimportant excitation characteristic. It indicates the frequency and thealteration in pitch frequency. Finally we may combine thepitch-frequency signal with the pitch-hiss-signal by assuming that thereis hiss in the absence of pitch. During intermission in speech, asystem-function is not found for the hiss spectrum, resulting in nooutput.

In accordance with the present invention a scan voice coding method andsystem is provided by performing a spectrum analysis of the speechspectrum, extracting a pitch-frequency signal therefrom including thealteration in its frequency, deriving a system-function, transmittingboth to a receiver for speech synthesizing.

More specifically, a communication system is provided by the presentinvention which achieves a substantial compression in bandwidth. In thissystem we derive by utilizing a pitch extractor at the input of atransmitter the larynx or pitch frequency. This frequency is thentransmitted to a receiver speech synthesizer and controls a pitchgenerator therein. This generator excites not only the pitch frequencybut also a large number of'its harmonies each having the same amplitude.At the input speech analyzer of the transmitter we also provide means todistinguish between hiss and larynx signals. These two events are alsotransmitted to the speech synthesizer located at the receiver and thesecontrol either a hiss or pitch generator. The aforementioned pitchfrequency is extracted by applying the principle that a vowel spectrumcontains harmonics of the pitch frequency. By the frequency differenceof these harmonics, pulses at the' pitch frequency are obtained.

There is simultaneously provided by the aforementioned communicationsystem a speech spectrum analyzer at the transmitter end. If a vowel ispresent at the input of the speech analyzer, this vowel will excitecorresponding voltages at lter outputs of the said analyzer. At thespeech synthesizerrlocated at the receiver, there is created all theharmonics of the pitch frequency by the pitch extractor signal. Sincethe speech synthesizer excites all harmonics with equal amplitudes,control is exercised of the amplitude of the harmonics to realize anenvelope corresponding to the speech spectrum envelope present at theinput of the speech analyzer. This is achieved by scanning the harmonicsof the speech spectrum present at the analyzer and controlling theharmonics of the pitch frequency at the synthesizer by the scanningvoltages. If there is a consonant instead of a vowel to be transmittedthe only difference will be that the pitch generator will be replaced bya hiss generator, and thereby there is provided a system for reproducingthe original input speech spectrum at the synthesizer.

By the aforementioned scanning, the present invention providesmeans-.for deriving a system-function signal which requires only asingle channel for transmission. The procedure of envelope transmissionwithin a time interval is referred to hereafter as time multiplex.

The present invention also provides a system utilizing only fourparameters, viz., a hiss signal, a larynx signal, the pitch frequencyand a modulation envelope which corresponds to the instantaneousposition of formants. Thus, a special formant extraction is unnecessary.

Also in accordance with the invention a speech band of 3 kc. is analyzedby 100 magnetostriction filters, each of which has a bandwidth of 30c.p.s. The single output voltages of these filters are scanned by therotating arm of a 10G-contact mercury-jet switch, rectified, andfiltered through a low-pass filter having a cutoff frequency ofapproximately 200 c.p.s. The envelope curve thus obtained is transmittedto the receiver in a channel of about 200 c.p.s. The arm of SO-contactmercury-jet switch rotates synchronously with the analyzer switch anddistributes the envelope curve voltage to the bias inputs of 5i)modulators. These modulators control the harmonics of thepitch-frequency signal, or of the bias spectra, in the out put of 50pairs (100) magnetostriction filters. The rcsulting 'output of thesynthesizer is a frequency-band whose spectrum is similar to that of theoriginal speechband. Including transmission of the pitch signal, afrequency-band compression in the order of 10:1 is made possible.

A more completeunderstanding of Ithe invention may be gained from thefollowing description of one of its preferred embodiments, whenconsidered in conjunction with the drawings, in which:

Fig. l is a diagram of a scan voice coding system for achievingspeech-band compression in signal transmission;

Fig. 2 shows a typical spectrum identifiable with signicant points inFig. l;

Fig. 3 shows diagrammatically a novel pitch extractor utilized in thescan voice coding system of Fig. 1;

Fig. 4 shows the circuit for the modulator and demodulator utilized atthe receiver-synthesizer of the scan voice coding system of Fig. 1; and

Fig. 5 is a graph illustrating the effect of shifting the phase of theoutput of three groups of modulators and then adding them.

Referring now to Fig. l, there is generated at the -output of microphone1 electric voltage fluctuations of sound pressure present at the inputof microphone 1. For example when speech is present, a characteristicspeech spectrum of about 3 kc. is present at point I. rfhis isillustrated by Waveform I in Fi". 2 showing the speech spectrum ofvoiced and unvoiced sounds. The voiced sound creates a speech spectrumcontaining a large number of harmonics of the pitch frequency and alsoan envelope of varying amplitude as shown at Ia of Fib. 2. The unvoicedsounds cause a noise spectrum as illustrated at Ib of Fig. 2.

From point I of Fig. l the signal is fed by way of line 2 to pitchextractor 3. This pitch extractor is more fully illustrated in Fig. 3.In the pitch extractor we utilize the fact that a vowel spectrumcontains harmonics of the pitch frequency and by the frequencydifference of these harmonics, pulses at the pitch frequency can beobtained.

Now referring to Fig. 3 for a more detailed explanationof pitchextractor 3, at terminal 10 there is fed the speech signal. Where thereis a vowel spectrum present as shown in Iaof Fig. 2, the lower part ofthe spectrum is suppressed Zby 1000 Vc.p.s. cutoff high-pass filter sothat at the input of diode 2 there is present waveform 9. Dioderectifier 2 produces a smaller amplitude of the second harmonic comparedto 4the pitch frequency amplitude. At the input of amplifier 3 weconsequently have waveform 10. The output of amplifier 3 is fed into 80c.p.s. low-pass filter 4. The effect of diode 2 is enchanced by passingthe fundamental and second harmonic through 80 c.p.s. low-pass filter 4having an attenuation slope of 12 db/octave. Thus the second harmonicwill be satisfactorily suppressed. At the input of clipper 5 there ispresent waveform 11 containing pulses representative of the fundamentalpitch. The input of integrator 6 receives normal pulses as shown inwaveform 12, these pulses are of uniform amplitude because of clipper 5.The output signal of integrator 6 as shown in waveform 13 is integrated.Diode 7 produces a signal as shown in waveform 14. These pulses are fedto pulse generator 8 whose output is a pulse representative of the pitchfrequency and which supplies the excitation function transmitted to thesynthesizer shown in Fig. 1.

Again referring to Fig. l, the output of pitch extractor 3 as indicatedat point V will be the envelope waveform at V of Fig. 2. From point I ofFig. 1, the speech spectrum of about 3 kc. is fed to push-pull modulator5 by way of line 4. There is also fed into push-pull modulator 5 by wayof line 6 a carrier frequency of 130 kc. which is generated inoscillator 7. The output at point II of Fig. 1 is fed simultaneously tofilters 101-200 by way of line 8. Since push-pull modulator 5 isutilized, the audio frequency band is shifted to the higher positionwhere the carrier (130 kc.) and the lower sideboard are suppressed. Theresulting frequency range of 130 to 133 kc. is analyzed bymagnetostriction filters 101 through 200. Each of the filters has abandwidth of 30 c.p.s. They are designed so that they overlap each otherat their half power points. The entire upper band Vextends from 130 to133 kc. The output of each of these magnetostriction filters 101-200 isfed to its corresponding rectifier 201-300. The resulting D.C. outputvoltages from rectifiers 201-300 are fed into its corresponding storagecapacitors 301-400. The output of storage capacitors 301-400 is fed byway of lines 401-500 to their corresponding connections 501-600 on100-contact-mercury-jet switch 9. Rotary arm 10 of switch 9 scans theoutputs` of storage capacitors 301-400. The output signal at point IIIthus obtained is shown as a waveform at III of Fig. 2. The voltagecharges in storage capacitors 301-400 are scanned by switch 10 rotating30 times a second. The fundamental frequency of the so developedscan-pulse is 3000 c.p.s. The amplitudes correspond with the envelopeslope of the waveform at point III as illustrated at III of Fig. 2. Bymeans of low-pass filter 11, the fast alterations in the curve aresuppressed, only the smooth envelope remains at point IV of Fig. 1 asillustrated in waveform at IV of Fig. 2. The cuto frequency of low-passfilter 11 equals the bandwidth needed for the envelope transmission, atpresent this frequency is 200 c.p.s. The output signals at point IV asillustrated in waveform IV of Fig. 2 are to be transmitted to the speechsynthesizer (also referred to as receiver).

It may be seen that at the analyzer (which may also be referred to asthe transmitter) there has beerextracted coded signals representative ofspeech. These signals are illustrated at IV and V of Fig. 2. Thesesignals are transmitted to a speech synthesizer (also referred to as areceiver). This transmission may be by any known means such as telephoneline, or radio.

On 'the synthesizer side, at point A is present the incomingpitch-frequency signal as shown in waveform as at point V in Fig. 1.Multivibrator 12 or hiss-noise generator 13 is controlled by saidincoming pitch-frequency signal.

When a voice or unvoiced sound is present at point A the waveform thatis present is illustrated in V(a) of 6 Fig. 2. Duringintermissions inputas shownin V(b).of Fig. 2v.

The normal position of relay switch 16 is at point C.

speech there is 11o-out- When a pitch-frequency signal is present atpoint A,

`we have an input either of line spectrum B or a noise spectrum C.Modulator 17 is also fed a carrier frequency of 130 kc. which isgenerated in oscillator 1S. The upper side band (130 130 kc.) of theoutput spectrum at point D (as illustrated at D of Fig, 2) in which thecarrier frequency is suppressed will receive an analysis rbymagnetostriction filters 601-700. As shown in Fig. 4,` saidmagnetostriction filters are connected in pairs, for example 601-602resulting in 50 double-filters. The pair'is comprised of one filter 601adapted to respond to a positive input and the other 602 toa negativeinput. The 50 filter outputs are linked to corresponding modulators701-750. The control inputs 851-900 of each ofv modulators701-750receives its voltage from the points 751-800 of 50 contact rotatingswitch 19, scanning the envelope E as shown at IV of Fig. 2.

This envelope voltage curve is representative of the modulator curve ofthe original speech spectrum. Arm

20v of switch 19 synchronously rotates with rotating arm` 10 of switch 9of the analyzer. Thus the envelope curve voltage is sampled for 1,6500second, but remains at the input of each of modulators 701-750 by meansof storage capacitors 801-850 until rotating arm 20 comes again to thispointrafter lf3() second. If, between samples, the envelope changes, themodulator gets the corresponding new voltage. l

The outputs of modulators 701-750 are connected so that the output ofmodulators 701, 704, 707 749 are connected together, outputs 702, 705750 are interconnected and 703, 706 '748 are also linked together.Thereby establishing a grouping of modulators 701-750 having threeoutputs 21-23 which are fed to a phase circuit 24 which will shift thephase 0, 120, and 240 respectively and then the resultant outputs arevadded in circuit 24. By these means, the modulation effect of anymodulator practically is restricted to the frequency range of thecorresponding filter as shown in the curve of Fig. 5. This is veryimportant since otherwise the envelope of the side band present at pointF of Fig. 4 (illustrated at F of Fig. 2) will be malformed and highdistortions appear. The complex output voltage of three groups is fed tomixer 25. Mixer 25 also receives a carrier frequency of kc. The complexvoltage of the three groups is mixed with the carrier and at point G ofFig. 4 there is present an output with the waveform as illustrated at Gof Fig. 2. This voltage output is fed to demodulator 26 from which wereceive a demodulated output at point H which is illustrated at H ofFig. 2. The audio frequency band at point H is very similar to the oneat point I of Fig. 1. We, thereby, reproduce at the output of thedemodulator a waveform which is similar to the original speech spectrumpresent at the input of the analyzer.

In effect we have at point IV of Fig. 1 derived a modulation curve. Thismodulation curve has been transmitted to the synthesizer. The pitchfrequency and its equal amplitude harmonics are controlled by filterswhich are similar to that at the analyzer. The switched frequencies atthe synthesizer are injected into a number of modulators (correspondingto number of pairs of filters). The modulators are also controlled byvoltages corresponding to the instantaneous position of the scannerwhich runs synchronized to the scanner which is located at the analyzer.When there is transmitted to the synthesizer scanner the voltages beingscanned at the analyzer, there is 7 obtained from 'the modulators theoriginal input spectrum. If there is a consonant instead of a vowel to be transmitted the only difference will be that the pitch generator willbe replaced by a hiss generator. y t

Many variations and modifications ofthe invention will occur to thosewho are skilled in the art to which the invention relates, and it isaccordingly intended that the claims that follow shall not be limited,but only illustrated, by the details of the embodiment of the inventionshown and described herein.

What is claimed is: y

l. A communication system including a speech analyzer and synthesizer,said system being comprised of transducer means located at said speechanalyzer to convert input speech into electrical signalsV representativeof the speech spectrum, means to extract from said electrical signalspitch frequency pulses yequal to the differencer between the harmonicand basic frequency components of said electrical signals, means tosimultaneously extract the modulation envelope from said electricalsignals, said envelope containing only direct current voltagecomponents, time multiplex means to transmit said direct current voltageenvelope in the form of pulses on a single frequency channel, meanslocated at said synthesizer to generate noise, means at said synthesizerto generate pulses having an audiorate of repetition, means controlledby said pitch frequency pulses toselect either said audio pulse means orsaid noise generating means, modulator means adapted to receive theoutput signals from said noise generating means or said audio pulsegenerating means, said modulating means simultaneously receiving a fixedfrequency carrier signal, means to divide the modu' lated signal fromsaid modulating means, means to arnplitude modulate said dividedsignals, said amplitude modulating means controlled by said transmitteddirect current voltage envelope, and transducer means to combine saidamplitude modulated signal to reproduce speech.

2. A communication system as defined in claim l wherein said means toextract pitch frequency pulses is comprised of first filter meansadapted to receive said` electrical signals, means to rectify the outputsignal from said first filter means, second means to filter saidrectified signal, means to clip said filtered rectified signal, means toVintegrate said clipped signal, means to rectify said inytegratedsignal, and pulse generator means controlled by said rectifiedintegrated signals.

3. A communication system as defined in claim l wherein said means toextract the modulation envelope from said electrical signals iscomprised of means to push-puli modulate said electrical signals with acarrier signal thus obtaining lower and upper sideband signals from saidmodulation operation, means to analyze said modulated signal utilizing aset of magnetostriction filters, said filters so arranged so that eachof said filters overlaps the next succeeding one at the half powerpoint, means to rectify the output signal of each of saidmagnetostriction filters to obtain a direct current voltage, means tostore direct current voltage, means to scan each of said stored voltage,and means to filter said scanned voltage.

4. A communication system including a speech analyzer and synthesizer,said system being comprised of means located at said speech analyzer toconvert inputV speech containing formants into electrical signalsrepresentative of the speech spectrums, means to extract from saidelectrical signals pitch frequency pulses equal to the differencebetween the harmonic and basic frequency components of said electricalsignals, means to simultaneously extract from said electrical signals amodulation envelope which corresponds to the instantaneous position ofsaid formants in said speech input, said envelope con taining onlydirect current voltage components, time multiplex means to transmit saiddirect current voltage envelope on a single frequency channel, meanslocated at said synthesizer to generate noise, means at saidsynthesizerto generate pulses havingan audio rate ofrepdesses tiltion,said rate of repetition controlled by said pitch fref quency pulses,means controlled by said pitch frequencyY receive the output signalsfrom said noise generating means or said audio pulse generating means,said modulating means simultaneously receiving a carrier signal having afixed frequency higher than any audio frequency, means to divide themodulated signal from said modulating means, means to amplitude modulatesaid modulated divided signals, said amplitude modulated meanscontrolled by said transmitted direct current voltage envelope, andtransducer means to combine said amplitude modulated signals toreproduce speech.

`5. A communication system comprising a speech analyzer and synthesizer,said analyzer adapted to receive at its input speech, means to convertsaid speech into electrical signals representative of the speechspectrum,

means to extract from said electrical signal pitch-pulsesignalsrepresentative of the fundamental pitch frequency offsaid speechspectrum, means to simultaneously modulate said electrical signals witha carrier signal of higher frequency than audio in such a manner as toproduce an upper Vand lower sideband signal, means to divide said uppersideband signal by a set of narrow bandpass filtersy to obtain at theoutput of each of said filters a voltage proportional to itscorresponding instantaneous speech spectrum, means to rectify each ofsaid filter output voltages to obtain a direct current voltage, means tostore each of Vsaid direct current voltages, means to scan said storeddirect current voltages, means to filter said scanned voltages to obtainonly the smooth envelope of said speech spectrum, means to transmit saidpitch pulse signals and said scanned direct current voltages, pulsegenerating means located at said speech synthesizer controlled by saidpitch pulse signals in such manner as to generate pulses having an audiorate, noise generating means normally connected to push-pull modulatormeans in the absence of said transmitted pitch pulse signals, relaymeans to connect said audio pulse generating means to said modulatormeans in place of said noise generating means, said relay meansoperative upon receipt of said transmitted pitch pulse signals, means togenerate' a corner signal for said pushpul1 modulator means, saidcarrier means generating a signal with a frequency equal to Vthat ofsaid carrier in said speech analyzer, means to divide the output signalfrom said push-pull modulator means by a set of narrow band filters,means to modulate each of said divided signals by said transmittedscanned direct current voltages, and means to combine said modulatedsignals to reproduce said speech present at the input of said speechanalyzer.

6. A communication system as dened in claim 5' wherein the means toextract said pitch pulse signals is comprised of filter means adapted toreceive said electrical signals representative of speech, means torectify the signal, resulting from said filtering operation. means tofilter said rectied signal, means to clip said filtered rectifiedsignal, means to integrate said clipped signal, means to rectify saidintegrated signal, and pulseV gen-k erator means controlled by saidrectified integrated signal.

7. A communication system as defined in claim 5 wherein the means todivide said upper sideband signal4 is comprised of a set ofmagnetostriction filters, each of said filters being arranged so thatthe succeeding filter overlaps at the half power point.

8. A communication system as defined in claim 5 wherein said means tocombine said modulated signals to reproduce speech is comprised of aphasing circuit adapted to receive three input signals from said dividedsignal modulators, said phasing circuit operating to provide 0 phaseshift for the first of said input signals, a D phase shift for thesecond of said input signals, and a 240 phaseY s'hiftfor thewthirdofpsaid input signals, mixer ,means receiving the signal output fromsaid phasing means and simultaneously said carrier signal gene1-ated insaid speech synthesizer, demodulator means References Cited in the le ofthis patent UNITED STATES PATENTS 2,098,956 Dudley Nov. 16, 1937 i0Dudley Mar. 21, 1939 Dudley May 27, 1941 Craib f. June 11, 1946Steinberg Apr. 14, 1953 Miller Apr. 5, 1955

